The present invention relates to a BLSR (bi-directional line switched ring) network of a SONET (Synchronous Optical Network), and more particularly to a BLSR network system having characteristics in the slot control.
In the Ring network, transfer of traffic is performed in a frame unit called, for example, a STS-1 (Synchronous Transport Signal-1). These frames are time-division multiplexed into a predetermined position of the time slot, then being transmitted. At present, as described in xe2x80x9cSONET Automatic Protection Switchingxe2x80x9d, ANSI (American National Standards Institute, Inc.), T1.105.01, there exist a 2-Fiber BLSR and a 4-Fiber BLSR as the BLSR network.
Concerning overview of the BLSR network, the description thereof is given in, for example, xe2x80x9cSONET BLSR Equipment Generic Criteria, Overview of the BLSR Architecturexe2x80x9d, GR-1230-CORE, Issue Dec. 3, 1996, Bell Communications Research Inc., pp. 3-1 to 3-24, 6-3, and 6-15 to 6-20.
In the 2-Fiber BLSR, the respective nodes are connected by two optical fibers, and a capacity within the respective channels is divided into two areas and the one is used for a working function and the other is used for a protecting function. In contrast to this, the 4-Fiber BLSR is configured in such a manner that there are provided working channels and protection channels and the respective nodes are connected by four optical fibers.
Both of the 2-Fiber BLSR and the 4-Fiber BLSR are systems in which under normal conditions, the traffic is transmitted using the working channel, and when there occurs a failure or the like, the traffic is protected using the protection channel. Hereinafter, taking as an example a 4-Fiber BLSR of an OC (Optical Carrier)-48, the explanation will be presented. Also, hereinafter, the traffic is described as xe2x80x9cpathxe2x80x9d.
FIG. 1 shows a configuration example of the BLSR network and an examples of use of the channels. In FIG. 1, reference numeral 10 denotes the entire system of the BLSR network. The BLSR network 10 includes optical fiber transmission lines 11 having a high signal transmission rate of, for example, 2.4 to 10 Gbits/sec and a plurality of nodes 12. The BLSR network illustrated in FIG. 1 includes six nodes (nodes A, B, C, D, E, and F).
With two fibers provided for each of the directions in the signal transmission, the optical fiber transmission lines 11 consist of bi-directionally four optical fibers. Concretely speaking, the optical fiber transmission lines 11 include a CW direction (i.e. a clockwise direction in the drawing) of working channel 13 and a CW direction of protection channel 14, and a CCW direction (i.e. a counterclockwise direction) of working channel 15 and a CCW direction of protection channel 16.
The plurality of nodes 12 are inserted into the optical fiber transmission lines 11 with a span placed therebetween. Each of the nodes 12 houses a lower-level network element (omitted in, the drawing) having a low signal transmission rate of, for example, 150 Mbits/sec or 600 Mbits/sec. The each of the nodes 12 performs an addition or a drop of the path (the STS-1) in the respective channels between the lower-level network element belonging thereto and the optical fiber transmission lines 11. Accordingly, the nodes 12 are also referred to as ADMs (Add Drop Multiplexers).
The example illustrated in FIG. 1 indicates that the STS-1 path, which is added at the node C and passes through the nodes D, E and is dropped at the node F, is transmitted using a time slot number #1 of the CW direction of working channel 13.
In the BLSR network system in FIG. 1, when there occurs a failure on only the working channel between, for instance, the node D and the node E, a path that is trying to pass through the failed span is transmitted using the protection channel. FIG. 2 shows a configuration in this situation.
In FIG. 2, when there takes place a failure on the working channel 13 between the node D and the node E, the node D and the node E switch the path, which is contained in the time slot number #1 that has been transmitted by the working channel 13, so that the path will be transmitted using a time slot number #1 of the protection channel 14. This switching illustrated in FIG. 2 is referred to as xe2x80x9cSpan Switchingxe2x80x9d.
Also, in the BLSR network system in FIG. 1, when there take place failures on both of the working channel 13 and the protection channel 14 between the node D and the node E, the path that is trying to pass through the failed span, as illustrated in FIG. 3, is caused to be looped at the node D back to the counterclockwise direction of protection channel 16.
As illustrated in FIG. 3, when there take place the failures on both of the working channel 13 and the protection channel 14 between the node D and the node E, the node D loops and switches the path, which is contained in the time slot number #1 that has been transmitted by the working channel 13, so that the path will be transmitted in the counterclockwise direction using a time slot number #1 of the protection channel 16. At this time, the nodes C, B, A and F permit the time slot number of the protection channel 16 to pass through without interchanging it.
In the node E, the path is caused to be transferred from the time slot number #1 that has been transmitted by the protection channel 16 to the time slot number#1 of the CW direction of working channel 13. Then, the path is dropped at the node F. The switching performed at the node D or the node E in FIG. 3 is referred to as xe2x80x9cRing Switchingxe2x80x9d.
As is seen from the above-mentioned description, it is the nodes at the both ends of a failed channel (in the examples in FIG. 2 and FIG. 3, the node D and the node E) that execute the Span Switching or the Ring Switching. Also, as illustrated in FIG. 3, when the Ring Switching is executed, the nodes A, B, C and F enter a Full Pass Through state in which they permit the protection channel and K-byte, i.e. switching control information, to pass through.
Next, the explanation will be given below regarding a configuration of the nodes. FIG. 4 illustrates a configuration of the node 12. Since all the nodes on the BLSR network are of the same configuration, the configuration of any one node is illustrated as a representative. As described earlier, the node 12 is referred to as the ADM (Add Drop Multiplexer). In the FIG. 4, the node 12 leads in the following: The four channels as Fiber Channels (channels for the Ring), i.e. the CW direction of working channel 13, the CW direction of protection channel 14, the CCW direction of working channel 15 and the CCW direction of protection channel 16, and an Add Channel 27 for adding path transmitted from the lower-level network element 12-1, and a Drop Channel 28 for dropping path so as to output it to the lower-level network element 12-1.
An optical signals inputted from another node is received by an optical receiver (R) 21, and is inputted into an overhead processing unit 23 so as to undergo an overhead processing. The path the overhead of which has been removed is then inputted into a cross connect unit 20. The cross connect unit 20 performs a TSI (Time Slot Interchange) and a TSA (Time Slot Assignment) of the high rate-side path (the OC-48) and the low rate-side path (the STS-1), and the path inputted therein is divided into the respective directions in the frame unit of the STS-1.
The paths thus divided are each multiplexed, and undergo the overhead processing at the overhead processing unit 23, and are converted into optical signals by an optical transmitter (T) 22, then being outputted from any one of the CW direction of working channel 13, the CW direction of protection channel 14, the CCW direction of working channel 15, the CCW direction of protection channel 16 and the Drop Channel 28.
For instance, in the configuration illustrated in FIG. 1, the STS-1 path is added at the node C from the lower-level network element 12-1 through the Add Channel 27 illustrated in FIG. 4. xe2x80x9cThen, the STS-1 path is divided into the CW direction of working channel 13, i.e. an transmission line toward the node D, at the cross connect unit 20 through the overhead processing unit 23, and is multiplexed into a position of the time slot number #1, then being outputted.
Also, in accordance with an instruction from an OS (Operation System), i.e. an apparatus for controlling the entire system and a state of a transmission line such as a fiber interruption, a path switching control unit 25 illustrated in FIG. 4 determines whether or not the Span Switching or the Ring Switching is executed, then informing the cross connect unit 20 of the switching command. Receiving the switching command from the path switching control unit 25, the cross connect unit 20 performs a switching of the path in correspondence with the type of the switching command such as the Ring Switching, the Span Switching or the Full Pass Through.
Next, the explanation will be given below concerning a channel misconnection. In the network illustrated in FIG. 1, when there occurs a node failure at the node D as is indicated by a mark X in FIG. 5, the path, which is contained in the time slot number #1 from the node C to the node F illustrated in FIG. 1, is caused to be transferred to the protection channel 16 at the node C. Then, the path passes through the nodes B, A and F using the time slot number #1 of the CCW direction of protection channel 16. Moreover, the path is looped back at the node E in the same way as the case in FIG. 3 and is caused to be transferred to the CW direction of working channel #1, then being dropped at the node F. At this time, the Ring Switching has been executed at the node C and the node E.
FIG. 6 shows another example of use of the channels in the BLSR network illustrated in FIG. 1. The example in FIG. 6 indicates the following: A path, which is added at the node B and passes through the node C and is dropped at the node D, and a path, which is added at the node D and passes through the node E and is dropped at the node F, are both transmitted using a time slot number #2.
In the channel setting state in FIG. 6, if there occurs a node failure, at the node D as is the case with FIG. 5, connections of, the time slots are performed in the same way as the case in FIG. 5. This results in the following situation illustrated in FIG. 7: The path added at the node B is caused to be transferred to the protection channel 16 at th!e node C. Then, the path passes through the nodes B, A and F using the time slot number #2 of the CCW direction of protection channel 16. Moreover, the path is looped back at the node E and is caused to be transferred to the CW direction of working channel #2, then being dropped at the node F.
As a result, it turns out that the path dropped at the node F has been incorrectly connected with the path added at the node B. This means that there has occurred a misconnection of the path. In order to prevent the misconnection like this, the ANSI stipulates performing an operation of inserting a path AIS (Alarm Indication Signal) into a designated position within the path at the node C and the node E in FIG. 7 at which the Ring Switching is executed. The operation of inserting the path AIS is referred to as xe2x80x9cSquelchxe2x80x9d.
FIG. 8 shows a transfer diagram of the paths at the time when the examples of use of the channels in FIG. 1 and FIG. 6 exist simultaneously. The rear end of an arrow indicates a node at which a path is added, and the arrowhead indicates a node at which the path is dropped. In FIG. 8, a path 1PCF denotes the STS-1 path in FIG. 1, and paths 2PBD and 2PDF denote the two STS-1 paths in FIG. 6, respectively.
According to the ANSI, for the Squelch operation, each of the nodes holds the following two types of maps: A Ring Topology Map indicating an order of the node IDs within the Ring and a STS Squelch Map indicating at which node a path, which passes through a present node or is added or dropped at the present node, is added and at which node the path is dropped.
FIG. 9 shows an example of the Ring Topology Map of the BLSR network in FIG. 1. FIG. 9 indicates that, within the BLSR network, the nodes are located in the CW direction in the order of B, C, D, E, F, A. Although the 6 nodes are described in FIG. 9, the number of the nodes up to 16 is allowed in the BLSR network.
FIGS. 10(a)-(f) show examples of the STS Squelch Maps that each of the nodes A, B, C, D, E, and F holds when the channel setting is presented as illustrated in FIG. 1 and FIG. 6. In FIG. 10(e), for instance, the STS Squelch Map that the node E holds indicates that a path in a West side time slot number #1 is added at the node C and, similarly, a path in an East side time slot number #1 is dropped at the node F. The STS Squelch Map that the node E holds also indicates that a path in a West side time slot number #2 is added at the node D and a path in an East side time slot number #2 is dropped at the node F.
Next, the explanation will be given below regarding a method of executing the Squelch. For instance, as is shown in FIG. 5 and FIG. 7, when the node D falls in a state of a failure, the node C and the node E that are adjacent to the node D execute the Ring Switching. In the BLSR, the K-byte on a Line Overhead allows a Missing Node to be identified. The Missing Node is defined as a node that is disconnected when seen from a present node. Regarding the K-byte, refer to xe2x80x9cSONET BLSR Equipment Generic Criteria, Overview of the BLSR Architecturexe2x80x9d, GR-1230-CORE, Issue 3, December 1996, Bell Communications Research Inc., pp. 6-15 to 6-20.
In the cases in FIG. 5 and FIG. 7, when seen from, for example, the node C and the node E, the Missing Node is the node D. In these cases, however, the source node (Src) of the path in the time slot number #1 is the node C. Accordingly, the Squelch is not executed at the node E, and the path is connected as is illustrated in FIG. 5.
Meanwhile, the source node of the path in the time slot number #2 is the node D, i.e. the Missing Node. For this reason, the misconnection will occur if the path is connected just the way it is without any transaction. Accordingly, the Squelch (the insertion of the path AIS) is executed as is illustrated in FIG. 7.
In the BLSR, it is impossible to execute an interchange of a time slot of a path passing through a node (the Time Slot Interchange: hereinafter described as the TSI). When, in FIG. 1, the channel setting from the node C to the node F is presented, the time slot cannot be changed at the node D and the node E through which the path passes. Thus, it turns out that the time slot #1 continues to be used on the BLSR. Similarly, in the example of the channel setting presented in FIG. 6, the time slot cannot be changed at the node C and the node E, and thus it turns out that the time slot #2 continues to be used.
In the above-described BLSR according to the prior arts, the TSI is not executed regarding the path passing through the high rate-side Ring network. Moreover, no provision has been given regarding a method of protecting a path at the time when a necessity for the switching occurs, i.e., for example, at the time of a failure in the case of supporting the TSI. Also, the STS Squelch Map that each node holds is based on, as the precondition, the case of not supporting the TSI in the BLSR, and thus the STS Squelch Maps cannot be enough information in the case of supporting the TSI.
The present invention can solve the above-described problems. It is an object of the invention to provide a BLSR network that allows a path, which passes through a node in the BLSR network, to change the time slot. Also, it is another object of the present invention to provide a method of protecting a path at the time when the switching occurs.
A bi-directional line switched ring network system according to the present invention includes a plurality of optical fiber communications lines, a plurality of nodes connected by a the plurality of optical fiber communications lines in such a manner as to form a closed circuit, each of the plurality of nodes performing an addition and a drop of a path between an external communications apparatus and the optical fiber communications lines, or permitting the path on the optical fiber communications lines to pass through, or performing a change of a direction of the path, and a control unit located in each of the plurality of nodes so as to control the addition the drop, the pass-through and the direction change of the path, wherein when the path passes through each of the nodes, the control unit sets a time slot number of the path in being inputted into the node and a time slot number of the path in being outputted from the node independently of each other.
A method of protecting a path in the bi-directional line switched ring network system according to the present invention includes the steps of setting, when a path passes through a node, a time slot number of the path in being inputted into the node and a time slot number of the path in being outputted from the node independently of each other, and when a failure occurs on one of working transmission lines between adjacent nodes (span), executing a span switching that allows a path to be maintained by using a protection transmission line the time slot number of which is the same as a time slot number of the working transmission line.
Further, a method of protecting a path in the bi-directional line switched ring network system according to the present invention includes the steps of setting, when a path passes through a node, a time slot number of the path in being inputted into the node and a time slot number of the path in being outputted from the node independently of each other, and when failures occur on all of transmission lines between adjacent nodes, executing a ring switching that allows a path to be maintained by using a protection transmission line which is provided with the same time slot number as that of a working transmission line between the adjacent nodes and the direction of which is opposite to a direction of the working transmission line.
Still further, a method of protecting a path in the bi-directional line switched ring network system according to the present invention includes the steps of setting, when a path passes through a node, a time slot number of the path in being inputted into the node and a time slot number of the path in being outputted from the node independently of each other, and when a failure occurs at one node, executing, at the other node connected with the failed node, a ring switching that allows a path to be transferred from a working transmission line or an external communications apparatus to a protection transmission line and by the ring switching, transferring the path to a protection transmission line which is provided with the same time slot number as a time slot number of a working transmission line between the connected nodes before the failure and the direction of which is opposite to a direction of the working transmission line, and executing, at the other node connected with the failed node, a ring switching that allows a path to be transferred from a protection transmission line to a working transmission line or an external communications apparatus, and by the ring switching, transferring the path with the time slot number of the protection transmission line.
In the embodiments of the present invention, in order to embody the above-mentioned operations, in association with a path to be received or transmitted on a working transmission line, the respective nodes, i.e. transmission units on the BLSR, hold a table indicating in sequence the time slot numbers used on the BLSR up to a transmission unit at which the path is terminated.